Commercial building heating, ventilating and air conditioning (HVAC) systems typically utilize automatically controlled heating and cooling valves to properly zone and circulate heated or chilled liquid to heat or cool the building, respectively. These heating and cooling valves may be ball valves, which have a ball-shaped valving member that is rotated by an actuator to selectively allow or prevent the flow of liquid therethrough. The actuator typically utilizes a small electric motor whose output is coupled through a speed reducing, torque multiplying gear train to rotate the valving member between its open and closed positions.
The actuators in such a system also include a fail safe mechanism that will properly position the valving member of the heating and cooling valves into a known state upon loss of electric power to the actuator. Typically, this fail safe mechanism is in the form of a spring return mechanism. During operation, the spring is wound during operation of the motor when the valving member is rotated from one position, i.e. its fail safe position, to the other. If electric power is lost at any time that the valving member is not positioned in its fail safe position, the spring operates to rotate the valving member back to its fail safe position.
Unfortunately, while the speed at which the spring is wound and unwound is controlled by the motor during normal operation, upon a loss of electric power, the driving speed of the spring return is limited only by the gear train and valve resistance. Since the gear train and valve resistances are designed to be low to increase efficiency during normal operation, this driving speed may become excessive. This excessive speed can destroy the gear train due to impact when the output gear contacts the stop at the end of the rotation.
One electro-mechanical actuator that overcomes this problem is disclosed in U.K. Patent Appln. G.B. 2,221,274 A entitled “Electro-mechanical actuators” filed on Oct. 21, 1987. This electro-mechanical actuator utilizes a wound coil spring to return the output member to a fail safe position when the electric supply to the electric motor fails. Unlike prior actuators, however, the release of energy by the coil spring is controlled by an air brake. This air brake utilizes a gear train mounted on the valve drive rod to spin a paddle mounted on a spindle. When the coil spring releases its energy upon a failure of the electric supply to the electric motor, the spring rotates the valve drive rod which, through the air brake drive train, rotates the paddle member. The rotating paddle member acts on the ambient air to provide a braking action or increased resistance, which controls the speed at which the valve is returned to prevent damage.
Unfortunately, the above-noted electro-mechanical actuator has drawbacks. For example, the electro-mechanical actuator requires two gear trains, namely a normal gear train for driving the output member and a parasitic gear train for driving the paddle. Because two separate and distinct gear trains are required, the number of components that need to be purchased, assembled, maintained, and potentially replaced is increased. Therefore, the cost of constructing and operating the electro-mechanical actuator is likely substantially increased.
In addition, the parasitic gear train in the above-noted electro-mechanical actuator includes gears progressing from the valve drive rod to the paddle wheel in a configuration that increases the rotational speed to the paddle. Therefore, the paddle spins fast and provides a breaking force against the rotation of the valving member even when the electric motor is driving the valving member under normal operation. As a result, the paddle wheel undesirably supplies the braking action to the electric motor, reducing the efficiency of the motor. In other words, the braking force of the paddle wheel counteracts the driving force of the electrical motor even though the paddle wheel was really included in the electro-mechanical actuator to control the release of energy from the coil spring.
There exists, therefore, a need in the art for an electro-mechanical actuator that overcomes one or more of the problems present in the art. The invention provides such an electro-mechanical actuator. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.